It is generally assumed that the ideal distribution for the lateral or vertical distribution of aircraft on neighbouring tracks is as in Fig. 1, where A and B are the centre lines of the routes or altitudes. Navaids and altimeters seem to be designed with this distribution as the ideal. Where air traffic control is imperfect and there is an appreciable risk that two aircraft without longitudinal separation will be at the same nominal altitude on the same route, then I suggest that neither the distribution of Fig. 1 nor that of Fig. 2 is ideal but that Fig. 3 represents the ideal because it minimizes the risk of collision due to air traffic control failure. This hypothesis has implications for the design of navaids and altimeters and for the specification of vertical and lateral separation procedures.